The neuronal basis of hyperkinetic movement disorders has long been unclear. We now test the hypothesis that changes in the firing pattern of neurons in the globus pallidus internus (GPi) are related to dyskinesias induced by low doses of apomorphine in patients with advanced Parkinson's disease (PD). During pallidotomy for advanced PD, the activity of single neurons was studied both before and after administration of apomorphine at doses just adequate to induce dyskinesias (21 neurons, 17 patients). Following the apomorphine injection, these spike trains demonstrated an initial fall in firing from baseline. In 9 neurons, the onset of 'on' was simultaneous with that of dyskinesias. In these spike trains, the initial fall in firing rate preceded and was larger than the fall at the onset of 'on' with dyskinesias. In three neurons the onset of 'on' occurred before that of dyskinesias, and the firing rate did not change at the time of onset of dyskinesias. Following injection of apomorphine, dyskinesias during 'on' with dyskinesias often fluctuated between transient periods with dyskinesias and those without. Average firing rates were not different between these two types of transient periods. Transient periods with dyskinesias were characterized by ISI independence, stationary spike trains, and higher variability of interspike intervals (ISIs). A small but significant group of neurons demonstrated recurring ISI patterns during transient periods of 'on' with dyskinesias. These results suggest that mild dyskinesias resulting from low doses of apomorphine are related to both low GPi neuronal firing rates and altered firing patterns.